The hydroformylation reaction, also known as the oxo reaction, is used extensively in commercial processes for the preparation of aldehydes by the reaction of one mole of an olefin with one mole each of hydrogen and carbon monoxide. One use of the reaction is in the preparation of normal- and iso-butyraldehyde from propylene. The ratio of the amount of the normal aldehyde product to the amount of the iso aldehyde product typically is referred to as the normal-to-iso (N:I) or the normal-to-branched (N:B) ratio. In the case of propylene, the normal- and iso-butyraldehydes obtained from propylene are in turn converted into many commercially valuable chemical products such as, for example, n-butanol, 2-ethyl-hexanol, n-butyric acid, iso-butanol, neo-pentyl glycol, 2,2,4-trimethyl-1,3-pentanediol, and the mono-isobutyrate and di-isobutyrate esters of 2,2,4-trimethyl-1,3-pentanediol.
In most cases, a phosphorus ligand-containing catalyst is used for the oxo process (so called “low pressure hydroformylation process”). Phosphorus ligands not only can stabilize the metal, but can also regulate the catalyst activity and selectivity. Oxo catalyst activity often decreases as the amount of phosphorus ligands increases while the catalyst stability increases with increasing amounts of ligand. Therefore, there exists an optimum concentration of phosphorus ligands for operating an oxo reactor which is the result of a tradeoff between increased stability and reduced catalyst activity.
In reality, the gradual loss of phosphorus ligands is inevitable because of decomposition and other reasons. In some cases, such as overflow reactors, the decomposition of ligands may be worsened due to the high temperature required to separate catalysts from products. In practice, fresh phosphorus ligands have to be replenished to the reactor on a regular basis to compensate for the loss of ligands.
Thus, there is a need in the art for ligands that not only stabilize the catalyst at higher concentrations, but can also increase the metal catalyst activity at such concentrations.